In situ synchrotron X-ray scattering and fluorescence techniques were used to simultaneously observe the evolution of the strain and composition of a growing crystal surface in real time. Control of the X-ray incidence angle allows us to obtain high surface sensitivity. We studied metal organic chemical vapor deposition (MOCVD) of epitaxial PbZrxTi1-xO3 (PZT) onto SrTiO3 (001) substrates under various growth conditions. We observe a strong increase in Zr incorporation as strain relaxation occurs, consistent with the effect of compositional strain on the thermodynamic driving force for growth. (c) 2006 Elsevier B.V. All rights reserved.